Nitric acid is produced commercially by passing ammonia and air across a flat gauze woven from platinum-rhodium-palladium alloy wire. The ammonia, initially, is converted to nitric oxide over the precious metal gauze, and the nitric oxide is, subsequently, oxidized and absorbed to form nitric acid. The conversion efficiency of ammonia to nitric oxide is a function of temperature, pressure, velocity of gas stream, volume of catalyst, and purity of the ammonia and air streams. The ammonia oxidation to nitric oxide generates a large exotherm and raises the temperature of the catalyst in the rang of 810.degree. C. to 960.degree. C. During the ammonia oxidation process some of the precious metal is volatilized from the surface of the gauze wire. The rate of loss is dependent on the temperature, pressure, and flow rate of gases across the catalyst surface. The cost of the precious metal lost from the ammonia oxidation catalyst is significant part of the cost of operating a nitric acid plant.
In nitric acid production, the catalyst pack consists of 3 to 50 sheets of flat woven gauze. The conventional flat woven gauze is typically made with 80 mesh per inch and 0.003" wire. However, UK patent GB 2062486B discloses the use of a system where the diameter of the wire is reduced from the front to the back of the flat woven gauze pack resulting in maximum conversion efficiency with minimum precious metal content and therefore also minimum metal loss from the catalyst. Other recent disclosures include U.S. Pat. No. 4,863,893 which claims improved catalyst "lightoff" by the use of a high surface area per unit area of catalyst by the deposition of fine platinum particles onto the surface of the flat woven gauze. U.S. Pat. No. 4,863,893 is a variation of the technology of U.S. Pat. No. 3,470,019 which had a different method of deposition. Patent application EP 0 364 153 A1 claims the use of a flat knitted fabric of precious metal for the oxidation of ammonia to nitric oxide. Recently, a flat woven gauze of 70 mesh, 0.003" wire diameter was introduced to the market place.
The catalyst pack of elements or sheets of gauze of precious metals and its use to catalyze ammonia is disclosed in U.S. Pat. No. 4,412,859, U.S. Pat. No. 4,497,657 and U.S. Pat. No. 4,526,614, all of which are hereby incorporated by reference, in toto.
By curve to flat ratio is meant the ratio of that portion of an element of a catalyst (a sheet or gauze etc.) that is not in the base plane of the element to the portion that is in that plane. When curved, it is the ratio of curved section to the flat section.